In the context of the present invention super-hard materials are defined as those materials having a Vickers hardness of no less than 2000 kg/mm2. These materials include a range of diamond materials, cubic boron nitride materials (cBN), sapphire, and composites comprising the aforementioned materials. For example, diamond materials include chemical vapour deposited (CVD) single crystal and polycrystalline synthetic diamond materials of a variety of grades, high pressure high temperature (HPHT) synthetic diamond materials of a variety of grades, natural diamond material, and diamond composite materials such as polycrystalline diamond which includes a metal binder phase (PCD) or silicon cemented diamond (ScD) which includes a silicon/silicon carbide binder phase.
In relation to the above, it should be noted that while super-hard materials are exceedingly hard, they are generally very brittle and have low toughness. As such, these materials are notoriously difficult to process into a product after the raw material is synthesized. Any processing method must be sufficiently aggressive to overcome the extreme hardness of the super-hard material while at the same time must not impart a large degree of stress or thermal shock to the material which would cause macroscopic fracturing of the material due to its brittle nature and low toughness. Furthermore, for certain applications it is important that surface and sub-surface damage at a microscopic scale, such as microcracking, is minimized to avoid deterioration of functional properties which may result from such surface and sub-surface damage including, for example, optical scattering, increased optical absorption, decreased wear resistance, and increased internal stress resulting in a decrease in coherence time for quantum spin defects near the processed surface.
There is narrow operating window for achieving successful processing of super-hard materials and many available processing methods fall outside this operating window. For example, most processing methods are not sufficiently aggressive to process super-hard materials to any significant extent in reasonable time-frames. Conversely, more aggressive processing techniques tend to impart too much stress and/or thermal shock to the super-hard material thus causing cracking and material damage or failure.
Certain processing methods have operational parameters which can be altered so as to move from a regime in which no significant processing of a super-hard material is achieved into a regime in which processing is achieved but with associated cracking and damage or failure of the super-hard material. In this case, there may or may not be a transitional window of parameter space in which processing can be achieved without cracking and damage or failure of the super-hard material. The ability to operate within a suitable window of parameter space in which processing can be achieved without cracking and damage or failure of the super-hard material will depend on the processing technique, the size of any transitional operating window for such a technique, and the level of operation parameter control which is possible to maintain processing within the window of parameter space in which processing can be achieved without cracking and damage or failure of the super-hard material.
In light of the above, it will be appreciated that post-synthesis processing of super-hard materials is not a simple process and, although a significant body of research has been aimed at addressing this problem, current processing methods are still relatively time consuming and expensive, with processing costs accounting for a significant proportion of the production costs of super-hard material products.
Post synthesis processing may comprise one or more of the following basic processes:                surface processing to remove material from the surface of the as-grown super-hard material in order to increase surface flatness, decrease surface roughness, remove surface defects, and/or attain a target thickness for the super-hard material;        surface processing to achieve a fine surface finish where minimal material is removed from the super-hard product, i.e. polishing; and        cutting of the super-hard material into target shapes and sizes for particular product application.        
In principle there are two basic forms of mechanical surface processing: (i) a two-body process in which abrasive particles are embedded/fixed in one body which is moved against a second body to process the second body; and (ii) a three-body process in which one body is moved relative to a second body to be processed and free abrasive particles, constituting a third body, are disposed between the first and second bodies in order to achieving surface processing of the second body.
The latter three-body approach to surface processing is known as lapping and it is this approach which is conventionally used to remove macroscopic quantities of surface material from super-hard materials. Three-body lapping, as opposed to a two-body surface processing technique, is preferred for removing macroscopic quantities of surface material from super-hard materials as it has been found that lapping is more efficient at removing surface material from a super-hard material without imparting a large degree of stress or thermal shock to the material which would cause macroscopic fracturing of the material due to its brittle nature and low toughness. In contrast, when it is desired to achieve a fine surface finish without removing macroscopic quantities of material then a two-body processing technique may be utilized. As such, conventionally lapping is used to remove material from the surface of an as-grown super-hard material in order to increase surface flatness, decrease surface roughness, remove surface defects, and/or attain a target thickness for the super-hard material. Subsequently, if a fine surface finish is required, the super-hard material is polished and this may be performed using a two-body surface processing technique in which abrasive material is fixed in a polishing wheel such as via resin bonding. Polishing may also be achieved using an iron or steel wheel which is diamond impregnated and this is known as scaife polishing. Although scaife polishing generally utilizes free diamond abrasive particles these are of a small size relative to pores within the iron or steel wheel and are thus embedded/fixed into the wheel thus effecting a two-body processing as opposed to a true three body lapping process.
In addition to the aforementioned mechanical lapping and polishing techniques a number of other techniques have been proposed for processing super-hard materials including:                chemical techniques include etching techniques such as plasma etching using suitable gas chemistries including, for example, one or more of hydrogen, oxygen, argon (or other inert gases), and chlorine (or other halides)—an example of an etching technique for achieving low surface roughness diamond surface finishes is described in WO2008/090511;        chemo-mechanical processing (CMP) techniques which combine mechanical and chemical processing mechanisms utilizing CMP slurries including abrasive grit particles and chemical components which react with the surface of the super-hard material being processed to change the chemical composition of the surface making it easier to remove—such processes having being utilized for other materials and are now currently under development for super-hard materials such as those comprising diamond;        laser beam cutting/ablating—laser cutting is the industry standard for cutting of synthetic diamond products;        high energy particle beam cutting/ablating—electron beam cutting has been proposed for cutting diamond products in the past and has recently been adapted to cut super-hard materials at significantly faster rates when compared with laser cutting;        electric discharge machining (EDM)—this technique is useful for cutting electrically conductive super-hard materials such as boron doped diamond materials; and        focussed ion beam (FIB) surface processing—this technique is known in the art for processing super-hard materials such as diamond as discussed in more detail below.        
Bayn et al. have described the use of FIB processing for fabricating nanophotonic structures in single crystal diamond for quantum applications. They report that a FIB processed diamond surface can be treated with a hydrogen plasma followed by acid treatment and that this improves optical and luminescence properties of the FIB processed diamond surfaces. In contrast to the previous studies of plasma treatments that leave an hydrogen terminated surface, they report that the acidic etching following the hydrogen plasma treatment renders the surface oxygen terminated, which has been shown to enhance the formation of NV−. It is also reported that the post FIB processing steps decrease surface roughness by 25%.
In Shuji Kiyohara and Iwao Miyamoto 1996 Nanotechnology 7 270 doi:10.1088/0957-4484/7/3/017 a method of processing diamond materials is reported including reactive ion beam machining of diamond using an electron cyclotron resonance (ECR)—type oxygen source. It is reported that the machining rate increases with increase in ion energy, reaches a maximum rate at an ion energy of 300 eV, then decreases gradually with further increase in ion energy. Furthermore, the surface roughness of diamond before and after oxygen ion beam machining was evaluated using an atomic force microscope (AFM) and a scanning electron microscope (SEM). It was found that the surface roughness increases with increase in ion incident angle, and decreases with increase in ion energy.
In K Tsuchiya et al 1995 Nanotechnology 6 158 doi:10.1088/0957-4484/6/4/009 it is further reported that in order to improve surface roughness of lapped material ion beam machining can be performed using argon ions and oxygen ions. It is reported that application of oxygen ion machining after removing the oxidized surface layer was effective for decreasing surface roughness.
Whetten, Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, Volume: 2, Issue: 2, Page(s): 477-480 also reports the Etching of diamond with argon and oxygen ion beams.
The most appropriate surface processing technique will depend on the end application, the type of surface finish required for the end application, and commercial considerations including an evaluation of the cost of a particular processing technique versus the commercial value of the product obtain after such processing. Surface processing parameters of interest may include one or more of: roughness; flatness; curvature; surface/sub-surface crystal damage; speed; cost; precision; and repeatability.
Prior to discussing embodiments of the invention in more detail, it may be pertinent to clarify the distinction between flatness and roughness, particularly in the context of synthesis and processing of super-hard materials such as synthetic diamond materials. In this regard, a skilled person will understand that flatness and roughness are two different characteristics of a surface and particular applications will be sensitive to either one or both of these characteristics. For example, a smooth curved surface has low roughness but it not flat as illustrated in FIG. 1 whereas a rough non-curved surface may be flat but have a high degree of roughness as illustrated in FIG. 2. Roughness is generally the deviation of a surface from a smooth target profile measured on a microscopic scale relative to the scale of the surface area whereas flatness is generally the deviation of a surface from a smooth target profile measured on a macroscopic scale relative to the scale of the surface area. The two parameters are thus distinguished by the method of measuring deviations from a smooth surface profile with roughness being measured by a technique which determines deviations from the smooth surface profile at a microscopic scale and flatness/curvature being measured by a technique which determines deviations from a smooth surface profile at a macroscopic scale. For a wafer of material having two opposing surfaces then surface parallelism may often also be an important additional parameter.
In light of the above, it will be evident that a surface which has low roughness may still deviate significantly from a smooth target profile due to macroscopic deviations from the smooth target profile. For example, FIG. 3 shows a schematic illustration of a wafer of super-hard material which has a surface profile which is bowed from a targeted smooth flat configuration. Furthermore, if the target profile is flat then a low surface roughness surface can still deviate significantly from a smooth flat target profile due to non-perpendicular surface processing leading to a sloped or wedge-shaped profile as illustrated in FIG. 4. Similar deviations to those illustrated in FIGS. 3 and 4 for flat surface can also occur when a curved surface profile is desired.
Such deviations from a target profile may be caused by stress introduced into the super-hard material during synthesis and/or during surface processing which can lead to bowing in a super-hard material Furthermore, macroscopic deviations from a smooth target profile may also result due to non-uniform processing.
The aforementioned issues are typically more problematic for super-hard materials when compared to less hard materials for a number of reasons as discussed below.
First, synthesis conditions for super-hard materials are often extreme, e.g. very high pressures and/or temperatures, leading to stress in the synthesized super-hard material which can cause bowing.
Secondly, the extreme hardness of super-hard materials typically requires a large amount of energy to be imparted to process a surface of the material and this generates heat leading to the generation of thermal stress during processing which can again lead to bowing.
Thirdly, due to the extreme hardness of super-hard materials, abrasive particles can be broken down into smaller particles during processing of a super-hard material which can result in differential processing of, for example, central and outer regions of a wafer of super-hard material.
Fourthly, the extreme hardness of super-hard materials typically requires a large force do be applied to the super-hard material during processing and if this force isn't uniformly applied across the surface of the super-hard material during process a sloped or wedge-shaped profile can result.
In addition to the above, even if a smooth, flat surface can be achieved by a particular surface processing method, or a combination of surface processing methods, problems may still exist in terms of crystal defects/damage being imparted into the crystal surface and sub-surface. FIG. 5 illustrates a super-hard material having a smooth and flat surface but where microcracks have been formed in a surface region due to forces imparted during the processing of the surface to a high degree of smoothness and flatness. This is a particular problem for super-hard materials due to the high hardness and low toughness of such materials. Several different methods are available for measuring surface and sub-surface crystal damage. For example, one technique involves applying a revealing plasma etch to the processed surface which preferentially etches cracked or damaged regions to form etch pits which can then be counted to evaluate the density of defects at the processed surface.
In light of the above, it is evident that while it is desirable for many applications to form low roughness and highly flat or precisely curved surfaces without imparting defects/damage into the crystal structure there are many problems associated with forming such surfaces in super-hard materials.
Embodiments of the present invention are primarily concern with processing of diamond and related super-hard materials to extreme levels of flatness or precisely defined curvatures. Such extremely levels of precision in terms of flatness or curvature are desirable for certain optical applications and certain electronic applications. While extremely levels of precision in terms of flatness or curvature have been achieved to date for many other materials, the nature of super-hard materials in terms of their extreme hardness and low toughness makes the acquisition of such surface finishes in super-hard materials very difficult to achieve in practice. It is an aim of certain embodiments of the present invention to solve this problem. It is a further aim of certain embodiments of the present invention to achieve extremely levels of precision in terms of flatness or curvature for super-hard materials while also providing a processing method which is cost efficient and reproducible over relatively large areas. It is yet a further aim of certain embodiments of the present invention to achieve extremely levels of precision in terms of flatness or curvature for super-hard materials while also providing addition advantageous surface and sub-surface characteristics including low roughness and low crystal damage.